Intussusception occurs when a length of bowel, the 'intussusceptum', prolapses and invaginates (or telescopes) into an adjacent segment, the 'intussuscipiens'. Four types of intussusception are described: ileocolic; ileo-ileocolic; colocolic; and ileoileal. Ileocolic is the most frequent and occurs in 90% of cases.

Fig. 6.5a,b. CT of the abdomen in a 12-year-old girl with a perforated appendicitis. a A large collection of fluid in the abdomen with an appendicolith seen low in the pelvis. b Fluid extending throughout the abdomen with an associated air-fluid level

adult patients has shown no difference in sensitivity or specificity for appendicitis when using relatively low dose (30 effective mAs) against standard dose (100 effective mAs) (KeyzeR et al. 2004). In children it may be possible to lower the dose even beyond the typical 30 mAs. A recent study looking at focused CT in children found no difference in the sensitivity, specificity of positive predicted value, or negative predicted value of interpretation of images whether they were focused below the right lower pole or whether they include the whole abdomen (FeffeRmAN et al. 2001). Whilst practice varies it would seem a reasonable strategy when considering CT for appendicitis in children to only scan from

Fig. 6.4. CT of the lower abdomen showing a swollen appendix containing an appendicolith and with adjacent stranding of the intraperitoneal fat

The majority of symptomatic intussusceptions in children arise in the ileum due to mucosal oedema and lymphoid hyperplasia of Peyer's patches following viral gastroenteritis or upper respiratory tract infection. These so-called idiopathic intussusceptions occur predominantly at the ileocaecal valve (95%). A "lead point" (5%) may be the cause of the intussusception. Typical lead points include a Meckel's diverticulum, polyps, duplication cysts, suture granulomas, appendiceal inflammation, Henoch-Schonlein purpura, or occasionally inspissated meconium. Symptomatic ileocolic and ileo-ileocolic intussusceptions are generally idiopathic, rather than secondary to a lead point. There is a reported association between intussusception and malrotation / malfixation (Waugh's syndrome).

The peak incidence of idiopathic intussusception is between the ages of 3 and 9 months (40%) with a range between 3 months and 2 years (Carty 2002). Approximately 75% of cases are in children less than 2 years, and in children older than 2 years a cause for a secondary lead point should be sought. Idiopathic intussusceptions are more common in boys (male: female = 2:1). Incidence of idiopathic intussusception is often seasonal, being more common in the late spring and the autumn.

The clinical diagnosis of ileocolic and ileo-ileo-colic intussusception is not always straightforward. The classical clinical triad of abdominal pain, red current jelly stool and a palpable abdominal mass is present in fewer than 50% of children with this condition. The child will typically draw up his legs to the abdomen during bouts of colic, which may be associated with facial pallor and the passage of red current jelly stools. Clinical examination of the abdomen may be difficult in a distressed child. The child may be shocked and peripherally shut down at the time of presentation. However, conversely, some children may be pain free at the time of presentation with only a history of bloody stools to suggest the diagnosis.

6.2.1 Imaging

Historically, the AXR has been the first investigation in children presenting with suspected intussusception. The most frequent plain film findings are those of reduced large bowel gas and the presence of a mass (Fig. 6.6) (Ratoliffe et al. 1993). Other signs include the meniscus sign and the

Fig. 6.6. AXR of a patient with intussusception. Note the soft tissue mass of the intussusception in the right iliac fossa and the dilated small bowel loops of obstruction

target sign (Sargent et al. 1994; Lee et al. 1994). Exclusion of an ileocolic intussusception on AXR is based on the presence of gas and stool in the caecum (Sargent et al. 1994). However, in 45% of children aged less than 5 years the sigmoid colon is in the right lower quadrant of the abdomen and therefore sigmoid filled with air and stool can be misinterpreted for caecum. Also, sometimes loops of "pulled-up" small bowel will project over the right lower quadrant as a result of the bowel shortening secondary to intussusception. It has been shown that intussusception was correctly identified on AXR in only 45% of children with intussusception (Sargent et al. 1994). Hence, as the plain abdominal radiograph is unable to categorically confirm or refute the presence of intussusception in the majority of cases, although it may be helpful as part of the work-up for investigation of an acute abdomen, an alternative investigation will still be necessary to confirm intussusception. The investigation of choice is ultrasound, and it is suggested that in cases of suspected intussusception that the patient should proceed directly to ultrasound without an AXR.

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